Stardate:1672.1--A transporter malfunction separates Kirk into two people: a kind and gentle vacillitating version, and a strong-willed arrogant Kirk who gives in to his every bestial urge. When the divided Kirks start to die, Spock and Scotty must work to reverse the split and save him, as well as repair the transporter so they can beam up a stranded crew.

Goofs--During the showdown between Good Kirk and Evil Kirk on the bridge, Evil Kirk's scratch marks are suddenly shown on the right cheek instead of the left as they had been throughout the episode.

After the dog-like creature dies, Dr. McCoy orders an autopsy on the animal; the term he should have used is "necropsy", which is used to describe post-mortem examinations on animals.

Trivia--Kirk's alternate green wrap-around uniform was introduced so that the audience would be able to tell the difference between the good Kirk and the evil Kirk.

The original script called for Spock to karate chop Kirk to subdue him. Leonard Nimoy felt that this would be an uncharacteristically violent act for a peace-loving species like the Vulcans so he came up with a pincer-like grasp on the neck that has since become known as the Vulcan Nerve Pinch.

Space: the final frontier. These are the voyages of the starship Enterprise. Its five-year mission: to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before.